Abstract

Diffraction patterns produced by grazing scattering of fast atoms from insulator surfaces are used to examine the atom-surface interaction. The method is applied to He atoms colliding with a LiF(001) surface along axial crystallographic channels. The projectile-surface potential is obtained from an accurate density-functional theory calculation, which includes polarization effects and surface relaxation. For the description of the collision process we employ the surface eikonal approximation, which takes into account quantum interference between different projectile paths. The dependence of projectile spectra on the parallel and perpendicular incident energies is experimentally and theoretically analyzed, demonstrating the range of applicability of the proposed model.

Quantum diffraction of fast atoms scattered from the topmost layer of surfaces under grazing angles of incidence can be employed for the analysis of detailed structural properties of insulator surfaces. From comparison of measured and calculated diffraction patterns we deduce the rumpling of the topmost surface layer of LiF(001) (i.e., an inward shift of Li{sup +} ions with respect to F{sup -} ions). The effect of thermal vibrations on the measurement of rumpling is accounted for by ab initio calculations of the mean-square vibrational amplitudes of surface ions. At room temperature this leads to a reduction of the apparent rumplingmore » by 0.008 A. We then obtain a rumpling of (0.05{+-}0.04) A, which improves its accuracy achieved in previous work.« less

The interaction potential for the elastic diffractive scattering of low-energy He atoms from the highly corrugated LiF(001) crystal surface is derived from semi ab initio pair potentials in the framework of the recently developed Tang--Toennies potential model (J. Chem. Phys. 80, 3726 (1984)). In addition to the sum of all He atom-crystal ion two-body potentials the induced dipole potential caused by the electric field of the ion lattice is taken into account, leaving only one free parameter, the C/sub 6/ dispersion constant of the He--F/sup -/ interaction. By simple adjustment of this parameter, it is possible to fit all ofmore » the experimental bound states of the atom-surface potential well, to within experimental error. Diffraction probabilities calculated by the close coupling method with this potential are shown to be in good agreement with the available experimental results. Two different empirical potential models based on the Morse potential are also investigated, but do not provide as good a description of the bound states and diffraction intensities.« less